dsm iv tr

The title is incorrect. It is meant to say "largest chainwheel".
I tried to edit the title, but no luck. This is what happens when you post in the morning...

Hey, folks!

This is a bike with a 7-speed freewheel, the wheels are true, and I've readjusted both der.'s from scratch to be on the safe side. Shifting otherwise works fine on both f/r der.'s.

Shifting to the largest chainwheel from anything above 4th gear in the rear causes the chain to jump off the largest chainwheel to the outside instead of shifting properly.

This started happening after the bottom bracket was replaced. I think, though, that the BB length is not quite right and thus is causing the chainline to be way off - and consequently dump the chain on the outside of the chainwheels when it's located on the smaller sprockets (which are obviously further over than the larger ones) at the back.

Right now I have the front der. high limit screw turned in enough so it's only able to shift to the granny + middle ring. The chainline is okay for commuting use in 5 of the 7 rear sprockets. It's rideable, but a little.. slow.

Any second (third, fourth..) opinions on what could be going on?
Is there a way to measure proper spindle length for a particular frame?

Thanks!